Flat roofs are typically equipped with one or more drains to remove precipitation and other water that collects on the roof surface. Rapid removal of such water is essential as its weight places great stress on a roof structure. If the weight of water on a roof reaches a sufficiently high amount, collapse of the roof structure can occur resulting in extensive property damage as well as grave injury to the building's occupants.
Drain strainers have been used to prevent the clogging of roof drains by leaves, trash, and other solid materials which deposit on a roof surface. These solid materials are readily carried to a drain opening by the force of draining water. One commonly used type of drain strainer consists of a domed-shaped housing perforated with spaced multiple openings. Placed over the mouth of a drain opening of approximately the same perimeter, this type of drain strainer filters the draining water by permitting the water to flow through the multiple openings while blocking solid materials being carried by the force of the draining water.
The shortcomings of this type of drain strainer are several-fold. Because the drain strainer's perimeter approximates that of the drain opening, its filtering capacity is limited. As a consequence, only a relatively small amount of solid materials need accumulate around the outer surface of the drain strainer for the multiple openings to become sealed, and water flow to the drain opening thereby blocked. Once water flow is blocked, water will collect on the roof surface and pose the hazards of structural damage, including roof collapse, and bodily injury. Additionally, this type of drain strainer is often constructed of polyvinyl chloride or other polymer. Exposure to sunlight, temperature changes, and other weather conditions causes the polymer strainer to become brittle and weakened. Consequently, the degraded polymer will eventually fracture. If used on a ballasted roof where roofing material is secured by stone or other ballast, fracturing of the weakened polymer drain strainer can occur much more readily because the surrounding ballast, by the force of draining water, repeatedly impacts the drain guard. Once fractured, solid materials being carried by the force of draining water freely pass into and soon clog the drain opening.
In U.S. Pat No. 3,121,682, auxiliary drain protector was disclosed surrounding a primary protector of lesser periphery. However, both the primary and auxiliary drain protectors only had uniformly spaced sidewall openings and hence did not provide enhanced flow of draining water to a drain opening.